In recent years, in such an optical pickup apparatuses equipped with a laser light source used as a light source for reproducing information recorded in an optical disc and for recording information for an optical disc, a trend to shorten the wavelength of the laser light source has been advanced. For example, a laser light source with a wavelength of 390 to 420 nm, such as a blue-violet semiconductor laser, has been put in practical use. In the case of using an objective lens with the same numerical aperture (NA) with that for DVDs (Digital Versatile Disc), the employment of such a blue-violet semiconductor laser light source enables to record the information of 15 to 20 GB for an optical disc with a diameter of 12 cm. Further, in the case where the NA of an objective lens is increased to 0.85, it becomes possible to record the information of 23 to 25 GB for an optical disc with a diameter of 12 cm.
Examples of optical discs designed for use with the above-mentioned objective lens with an NA of 0.85, include BDs (Blue-ray Disc). Since a comma aberration occurring due to the inclination (skew) of an optical disc increases, the protective substrate of the BD is designed to be thinner (0.1 mm in contrast with 0.6 mm of a DVD) than that of the DVD, thereby reducing the comma aberration due to the skew.
Incidentally, only a performance capable of recording and/or reproducing information properly for BDs is not said to have sufficient commercial value as products of optical disc players/recorders (optical information recording reproducing apparatus). In consideration of a fact that, at present, DVDs and CDs (Compact Discs) storing various kinds of information have been on the market, only the performance capable of recording and/or reproducing information for BDs is not sufficient. For example, a performance capable of enabling to record and/or reproduce information properly also for DVDs and CDs possessed by users leads to enhance the commercial value as optical disc players/recorders for BDs. In view of such a background, optical pickup apparatuses installed in optical disc players/recorders used for BDs are required to have a performance capable of recording and/or reproducing information appropriately while maintaining compatibility for any one of BDs and DVDs and further CDs.
As a method capable of recording and/or reproducing information appropriately while maintaining compatibility for any one of BDs and DVDs and further CDs, a supposable method is to switch over selectively an optical system for BDs and an optical system for DVDs and CDs in response to the recording density of an optical disc for which information is recorded or reproduced. However, according to this method, a plurality of optical systems is needed, which is not advantageous in terms of miniaturization and leads to increase cost.
Accordingly, in order to simplify the structure of an optical pickup apparatus and to realize the reduction of cost, even in an optical pickup apparatus with compatibility, it is preferable to make an optical system for BDs and an optical system for DVDs and CDs into a common optical system so as to reduce the number of optical components constituting the optical pickup apparatus as much as possible. Therefore, a technique to make an objective lens to be arranged so as to face an optical disc into a common objective lens for each of optical discs different in type is the most advantageous for the simplification of the structure and for the reduction of cost for the optical pickup apparatus. In order to realize such a common objective lens for multiple types of optical discs designed for use with respective different wavelengths for recording and/or reproducing information, it is required to form a diffractive structure having wavelength dependency for a spherical aberration on an objective lens.
Patent Document 1 discloses an objective lens which includes a structure in which two basic structures with respective diffractive structures are superimposed on each other and can be used in common for three types of optical discs, and also discloses an optical pickup apparatus in which this objective lens is mounted.
Two of the three types of optical disc disclosed in Patent Document 1 are apparently DVDs and CDs. Further, with reference to examples, an NA is made 0.67 (NA=0.67) for a wavelength of 405 nm. That is, Patent Document 1 is said to be an application which places a weight on compatibility for three types of optical discs of HD-DVDs, DVDs, and CDs. The inventor studied the applicability of the examples of this application for the compatibility for three types of optical disc of BDs, DVDs, and CDs. As a result, it turned out that since the NA of BDs is generally 0.85 being a high NA as compared with HD-DVDs, a spherical aberration becomes large due to a change in the refractive index of an objective lens when temperature changes. Further, it also turned out that in the superimposition structure disclosed by Patent Document 1 in which diffractive structures are superimposed on each other, since the amount of a level difference in stepped portions in the optical axis direction increases, when wavelength changes, large fluctuation of diffraction efficiency tends to occur.